A. Field of Invention
This invention pertains to a method and apparatus for the manufacture of a toothed power transmission belt, and more particularly to a means of correcting the pitch of the belt dynamically, while the belt is being cured.
B. Description of the Related Art
Synchronous power transmission belts transmit motion and power by the engagement of teeth on the belt to teeth on driving and driven sprockets. Commercial tolerances for some of these belts are given in RMA Engineering Standards IP-24 and IP-27. The tolerance on tooth pitch length on such belts ranges from 4444 ppm for miniature belts of 91.44 mm pitch length to 291.5 ppm for large belts of 6860 mm pitch length. IP-27 specifies a pitch length tolerance of +−1.12 mm for a 200 tooth, 14 mm pitch belt, so the average tooth pitch tolerance is ±0.0056 mm. The pitch between adjacent teeth must be generally within ±0.0254 mm of the nominal pitch, and must be statistically distributed about the nominal value.
Statistical variation of material properties, thermal expansion, and dimensional variation of uncured belt components often result in cumulative error of pitch and tooth position when a synchronous power transmission belt is cured incrementally in a press. This cumulative error can result in excessive pitch error when a leading portion of the last section is cured to a trailing portion of the first section.
Press cured synchronous belts usually contain layers of fabric and elastomer that are formed into a flexible cylinder. A helical layer of one or more tensile members is also usually included. In a typical manufacturing process, the uncured and unmolded belts are supported on cylinders placed on either side of a planar mold. The cylinders maintain alignment of the belts to the mold during the sequential molding of belt sections, and also apply tension to the cords of the belt to prevent their displacement in the molding process.
The spacing of the teeth on the molded belt must be even all around the belt, even when the belt is shorter or longer than the nominal circumference. The exact circumference of the belt slab is difficult to measure accurately because the radius of the belt neutral plane on the cylinders is difficult to determine.
The present invention provides methods and apparatuses for avoiding large errors in tooth spacing independent of the actual circumference of the belt.